In 95% or above of conventional solar cells, semiconductors consisting of Si materials are used. However, Si semiconductors have a forbidden band gap of 1.1 eV; therefore, conventional solar cells have not achieved sufficiently-high energy conversion efficiency. Moreover, having a small optical absorption coefficient, Si semiconductors need to have a thickness of 100 μm or above for light to be absorbed sufficiently. Accordingly, such Si semiconductors are not suitable for the purpose of making thin solar cells.
There have been attempts to expand the forbidden band gap of silicon-based semiconductors in order to improve their energy conversion efficiency in the solar cells. In Japanese Unexamined Patent Application Publication No. 2005-294810 (Document 1), disclosed is a mixed crystal semiconductor thin film in which BaSi2 is doped with alkaline earth metal atoms, such as Sr atoms, Ca atoms, or Mg atoms. It is disclosed that a mixed crystal semiconductor thin film doped with Sr atoms as the alkaline earth metal atoms especially has a large optical absorption coefficient, and has a forbidden band gap of approximately 1.4 eV.
Meanwhile, it is known that an electron density in a semiconductor is increased by doping impurities. However, it has not been easy to adjust the electron density to a desired value over a wide range of electron density.